We consider the continuous wave propagation in a highly dispersive and nonlinear medium wherein the wave propagation is governed by the generalized nonlinear Schrödinger equation. We demonstrate the novel flat-top modulational instability gain for a wide range of modulation frequencies in the anomalous dispersion regime of highly dispersive and nonlinear media. We find that the resulting MI gain is independent of input power. Besides, we compare the analytical results with those of numerical results. © 2008 SPIE.